1. Field of the Invention
The present invention relates to an apparatus for projection display using liquid crystal displays (LCDs), and in particular, to an apparatus for projection display using LCDs that can be compacted by reducing the number of optical elements to be less than the conventional method of splitting and synthesizing colors by using an X-prism, and that safe performance of a projection display can be guaranteed by employing reflection type LCDs as a spatial light modulator (SLM).
2. Description of the Prior Art
With the recent trend of enlarging the scale of display, data projectors, projection TVs, projection monitors, etc. using a projection technology are being developed in a rapid speed. Recently, a reflection type LCD, which enhances aperture rates of a pixel by installing reflection type electrodes for each pixel, is employed in the projection display. The reflection type LCD is capable of enhancing aperture rate in comparison with the transparent LCD of the conventional type, and is suitable for projection display of high efficiency.
Unlike the conventional projection display using the transparent LCD, which employs a polarizing beam splitter (PBS) of a polarizer function and a PBS of an analyzer function, the projection display using the reflection type LCDs has both functions, thereby carrying an advantage of satisfying the need of compacting a system.
In applying the reflection type LCD, however, a single optical part has bi-functions of polarizer and analyzer. Therefore, the proportion of affecting the entire system by the performance of the part becomes fairly high. In particular, thermal stability of the part becomes problematic because about two times the optical energy than the method of using a transparent optical modulator passes through the optical part. The method conceived to solve this problem was to employ a material of high thermal stability for the optical part.
A method for dispersing the optical energy is to use one optical modulator and a color wheel. However, this method outputs only one of the R, G, B basic colors at one moment, thereby causing a principal optical loss of about 70%. Another method for dispersing the optical energy is to use a dichroic mirror before the illuminating light is incident to the optical modulator to separate the light into basic colors of R, G, B. The split light then works for the optical parts that have bi-functions of polizer and analyzer as well as to three reflection type optical modulators. This method has a problem of enlarging the system despite a satisfactory optical efficiency.
Under these circumstances, a need has arisen to conceive an apparatus for projection display using three reflection type optical modulators to enhance optical efficiency as well as a material of sufficient thermal stability while compacting a system by minimizing dispersion of the optical energy.
The outstanding prior art in this field is disclosed in U.S. Pat. No. 5,808,795.
In U.S. Pat. No. 5,808,795, three basic colors are split by a plate dichroic filter of X shape. The split colors are incident to three PBS groups for polarization, and modulated by three reflection type LCDs. The modulated light is synthesized by means of an X-prism, and projected to a screen through a projection lens. To remove deterioration of the quality of screen caused by an external or thermal stress, the PBS of the system is composed of a material having a photoelastic constant less than 1.5xc3x9710xe2x88x926 mm2/N.
FIG. 1 is a diagram illustrating an apparatus for projection display according to the prior art. The following is a description of an operation made with reference to FIG. 1.
A white light emitted from a light source is divided into three components of R, G, B by a plate dichroic mirror 11. The description of the following process will be made with reference to the G component only because the optical parts operable for R, G, B components are similar to one another.
The G component is incident to a PBS 15G, which is in charge of pre-polarizing, by means of a mirror 12G. The component pre-polarized by the PBS 15G is re-polarized by a PBS 14G to face a reflection type LCD 13G. The G component modulated by the reflection type LCD 13G is analyzed by the PBS 14G to face an X-prism 17. Here, the PBS is composed of a material having a photoelastic constant less than 1.5xc3x9710xe2x88x928 cm2/N.
The image synthesized with R, G, B components by the X-prism 17 is projected to a screen 200 through a projection lens 18.
Of the optical energy incident to the PBS, the energy absorbed to the PBS becomes a thermal stress of the PBS. An optical path difference AR which can estimate a degree of influence of the stress onto the performance of the PBS can be expressed by the following Equation 1.
Equation 1
xcex94R=(2Π/xcex)xc2x7xcex94nxc2x7L=(2Π/xcex)xc2x7Cxc2x7xcex94"sgr"xc2x7L
Here, xcex refers to a wavelength of the light, while xcex94n refers to an amount of birefringence. L refers to a thickness of the light path, and C refers to a photoelasticity of the material. xcex94"sgr" refers to a difference between two principle stresses perpendicular to an optical axis.
Therefore, the conventional apparatus described above attempted to secure stability of a system and eveness of a screen by limiting the material of the PBS to have a photoelastic constant less than 1.5xc3x9710xe2x88x928cm2 /N.
However, the prior art splits colors by using a dichroic mirror of X shape, and operates one optical component of three colors into the PBS for the purpose of polarization and analysis. The material of low photoelasticity is used for the PBS in major wavelength of three basic colors. This means that less optical energy is required for one PBS in comparison with the present invention. Since the prior art requires optical elements for each basic color, however, the number of optical elements is increased, thereby escalating the cost and size of the product.
Also, the prior art uses glass of adjusted photoelasticity, which contains a predetermined content ratio of PbO. Such glass has a drawback of low transmittance in a short wavelength of a visible optical area. When the optical energy working for the PBS is increased, the optical energy absorbed by the PBS is increased as well. As a consequence, stress is increased and brightness uniformity cannot be secured despite low photoelasticity.
Thus, the conventional apparatus poses problems of resulting in a great number of optical elements while failing to secure a stable and brightness uniformity.
It is, therefore, an object of the present invention to provide an apparatus for projection display using reflection type LCDs that can be compacted by reducing the number of optical elements to be less than the conventional method of splitting and synthesizing colors by using an X-prism, and that safe performance of a projection display can be guaranteed by employing reflection type LCDs as an SLM.
To achieve the above objects, there is provided an apparatus for projection display using reflection type LCDs according to an embodiment of the present invention, comprising an illuminating device including a lamp and a polarizing element for illuminating S wave or P wave; a color splitter/synthesizer including the reflection type LCDs for providing an image corresponding to an inputted image and converting a polarizing state of incident light when reflecting the light, a retarder stack for differentiating the polarized state of a predetermined color from the incident light, and polarizing beam splitters composed of a material having a photoelastic constant less than 0.03xc3x9710xe2x88x926 mm2/N at a wavelength of 589.3 nm and a minimum transmittance higher than 90% in transmitting the thickness of 25 mm at a wavelength ranged 0.42 xcexcm-0.70 xcexcm for performing color split, color synthesis and analysis; and a projector for projecting the image passed through the color splitter/synthesizer onto a screen.
The color splitter/synthesizer comprises:
a first retarder stack for differentiating and passing the polarized state of a predetermined color of the light, and passing the other colors of the light incident from the illuminating device; a first polarizing beam splitter for color splitting the light passed through the first retarder stack to transmit or reflect the same; a second polarizing beam splitter for transmitting the light passed through the first polarizing beam splitter to a first reflection type LCD, and analyzing the light reflected therefrom; a second retarder stack for differentiating and passing the polarized state of a predetermined color of the light reflected from the first polarizing beam splitter, and passing the other colors of the light; a third polarizing beam splitter for color splitting the light passed through the second retarder stack to transmit the same to the second and the third reflection type LCDs, and color synthesizing and analyzing the light reflected therefrom; a third retarder stack for differentiating and passing the polarized state of a predetermined color of the light synthesized by the third polarizing beam splitter, and passing the other colors of the light; a fourth polarizing beam splitter for synthesizing the light passed through the third retarder stack with the light analyzed by the second polarizing beam splitter; and a fourth retarder stack for differentiating and passing the polarized state of a predetermined color of the light synthesized by the fourth polarizing beam splitter, and passing the other colors of the light.